Cutter element with non-linear, expanded crest

ABSTRACT

A cutter element for a drill bit has a non-rectilinear crest. The cutter element has a base portion and an extending portion and the extending portion has either a zero draft or a negative draft with respect to the base portion. The non-positive draft allows more of the borehole bottom to be scraped using fewer cutter elements. The cutter elements having non-positive draft can be either tungsten carbide inserts or steel teeth.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims the benefit of 35 U.S.C. 111(b)provisional application Serial No. 60/057,915 filed Sep. 4, 1997 andentitled “Cutter Element with Expanded Crest Geometry” and is adivisional of application Ser. No. 09/146,095, filed Sep. 3, 1998 andentitled “Cutter Element with Expanded Crest Geometry”, both of whichare incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The invention relates generally to earth-boring bits used todrill a borehole for the ultimate recovery of oil, gas or minerals. Moreparticularly, the invention relates to rolling cone rock bits and to animproved cutting structure for such bits. Still more particularly, theinvention relates to a cutter element having an expanded crest geometrywhich extends up to and beyond the envelope of its base portion.

BACKGROUND OF THE INVENTION

[0003] An earth-boring drill bit is typically mounted on the lower endof a drill string and is rotated by rotating the drill string at thesurface or by actuation of downhole motors or turbines, or by bothmethods. With weight applied to the drill string, the rotating drill bitengages the earthen formation and proceeds to form a borehole along apredetermined path toward a target zone. The borehole formed in thedrilling process will have a diameter generally equal to the diameter or“gage” of the drill bit.

[0004] A typical earth-boring bit includes one or more rotatable cuttersthat perform their cutting function due to the rolling movement of thecutters acting against the formation material. The cutters roll andslide upon the bottom of the borehole as the bit is rotated, the cuttersthereby engaging and disintegrating the formation material in its path.The rotatable cutters may be described as generally conical in shape andare therefore sometimes referred to as rolling cones. Such bitstypically include a bit body with a plurality of journal segment legs.The rolling cone cutters are mounted on bearing pin shafts that extenddownwardly and inwardly from the journal segment legs. The borehole isformed as the gouging and scraping or crushing and chipping action ofthe rotary cones remove chips of formation material which are carriedupward and out of the borehole by drilling fluid which is pumpeddownwardly through the drill pipe and out of the bit.

[0005] The earth-disintegrating action of the rolling cone cutters isenhanced by providing the cutters with a plurality of cutter elements.Cutter elements are generally two types: inserts formed of a very hardmaterial, such as cemented tungsten carbide, that are press fit intoundersized apertures or similarly secured in the cone surface; or teeththat are milled, cast or otherwise integrally formed from the materialof the rolling cone. Bits having tungsten carbide inserts are typicallyreferred to as “TCI” bits, while those having teeth formed from the conematerial are known as “steel tooth bits.”

[0006] The cutting surfaces of inserts are, in some instances, coatedwith a very hard “superabrasive” coating such as polycrystalline diamond(PCD) or cubic boron nitride (PCBN). Superabrasive materials aresignificantly harder than cemented tungsten carbide. As used herein, theterm “superabrasive” means a material having a hardness of at least2,700 Knoop (kg/mm2). Conventional PCD grades have a hardness range ofabout 5,000-8,000 Knoop, while PCBN grades have a hardness range ofabout 2,700-3,500 Knoop. By way of comparison, a typical cementedtungsten carbide grade used to form cutter elements has a hardness ofabout 1475 Knoop. Similarly, the teeth of steel tooth bits may be coatedwith a hard metal layer generally referred to as hardfacing. In eachcase, the cutter elements on the rotating cutters functionally breakupthe formation to create new borehole by a combination of gouging andscraping or chipping and crushing.

[0007] The cost of drilling a borehole is proportional to the length oftime it takes to drill to the desired depth and location. In oil and gasdrilling, the time required to drill the well, in turn, is greatlyaffected by the number of times the drill bit must be changed in orderto reach the targeted formation. This is the case because each time thebit is changed, the entire string of drill pipe, which may be mileslong, must be retrieved from the borehole, section by section. Once thedrill string has been retrieved and the new bit installed, the bit mustbe lowered to the bottom of the borehole on the drill string, which.again must be constructed section by section. As is thus obvious, thisprocess, known as a “trip” of the drill string, requires considerabletime, effort and expense. Accordingly, it is always desirable to employdrill bits which will drill faster and longer and which are usable overa wider range of formation hardness.

[0008] The length of time that a drill bit may be employed before itmust be changed depends upon its rate of penetration (“ROP”), as well asits durability or ability to maintain an acceptable ROP. The form andpositioning of the cutter elements (both steel teeth and TCI inserts)upon the cone cutters greatly impact bit durability and ROP and thus arecritical to the success of a particular bit design.

[0009] Bit durability is, in part, measured by a bit's ability to “holdgage”, meaning its ability to maintain a full gage borehole diameterover the entire length of the borehole. To assist in maintaining thegage of a borehole, conventional rolling cone bits typically employ aheel row of hard metal inserts on the heel surface of the rolling conecutters. The heel surface is a generally frustoconical surface and isconfigured and positioned so as to generally align with and ream thesidewall of the borehole as the bit rotates. The inserts in the heelsurface contact the borehole wall with a sliding motion and thusgenerally may be described as scraping or reaming the borehole sidewall.

[0010] In addition to the heel row inserts, conventional bits typicallyinclude a primary “gage” row of cutter elements mounted adjacent to theheel surface but oriented and sized so as to cut the comer as well asthe bottom of the borehole. Conventional bits can also contain asecondary gage trimming row or a nestled gage row with lesser extensionto assist in trimming the bore hole wall. Conventional bits also includea number of additional rows of cutter elements that are located on thecones in rows disposed radially inward from the gage row. These cutterelements are sized and configured for cutting the bottom of the boreholeand are typically described as primary “inner row” cutter elements.Together, the primary gage and primary inner row cutter elements of thebit form the “primary rows.” Primary row cutter elements are the cutterelements that project the most outwardly from the body of the rollingcone for cutting the bore hole bottom.

[0011] A review of post run bit performance data from 1991 through 1995indicated that most aggressive roller cone cutting structures from bothmilled tooth and tungsten carbide insert bits were sub-optimal ataddressing very soft rock formations (i.e. less than 2000 psi unconfinedrock compressive strength). Ultra-soft to soft formations typicallyconsist of clays, claystones, very soft shales, occasionally limy marls,and dispersed or unconsolidated sands, typically exhibit plasticbehavior. Very soft or weak clays/shales vary in their mechanicalresponse from more competent (harder) shales, under the same compressionloads, as applied in rotary rock bit drilling. Soft shales respondplastically, or simply deform under the applied load, as opposed to abrittle failure or rupture (crack) formed in more competent rocks tocreate the cutting or chip. In these very soft/plastic formationapplications, we cannot rely on conventional brittle rock failure modes,where cracks propagate from the loaded tooth penetration crater to theadjacent tooth craters, to create a chip or cutting. For this reason,the cutting structure arrangement must mechanically gouge away a largepercentage of the hole bottom in order to drill efficiently. In thesetypes of formations, maximum mechanical efficiency is accomplished bymaximizing the bottom hole coverage of the inserts contacting the holebottom per revolution so as to maximize the gouging and scraping action.

SUMMARY OF THE INVENTION

[0012] The present invention provides maximum scraping action and allowsgreater flexibility in the number of cutter elements used on a drillbit. According to the present invention, at least one cutter element ona bit is provided with a non-positive draft. The term “draft” is used torefer to the relationship between the extending portion of the cutterelement and envelope defined by the cutter element base. Moreparticularly, the term “non-positive draft” is used to refer to cutterelements in which the extending portion of the cutting element extendsout to or beyond the envelope of the base portion. According to thepresent invention, the non-positive draft can take the form of either azero or a negative draft. The concepts of the present invention can beused in cutter elements that have non-circular or non-cylindrical basesand can be used in tungsten carbide inserts, in tungsten carbide insertscoated with superabrasive, and in steel teeth.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Other objects and advantages of the invention will becomeapparent upon reading the following detailed description and uponreference to the accompanying drawings in which:

[0014]FIG. 1 is a perspective view of an earth-boring bit;

[0015]FIG. 2 is a partial section view taken through one leg and onerolling cone cutter of the bit shown in FIG. 1;

[0016] FIGS. 3A-D are top, front, side and perspective views,respectively, of a prior art chisel insert;

[0017] FIGS. 4A-C are top, front, and side views, respectively, of aprior art conical insert;

[0018] FIGS. 5A-C are top, front and side views, respectively, of achisel insert constructed in accordance with a first embodiment of thepresent invention;

[0019] FIGS. 6A-D are top, front, side and perspective views,respectively, of a chisel insert constructed in accordance with a secondembodiment of the present invention;

[0020]FIG. 3E shows the cutter elements of a prior art drill bit rotatedinto a single plane;

[0021]FIG. 6E shows the cutter elements of FIGS. 6A-D rotated into asingle plane;

[0022] FIGS. 7A-C are top, front and side views, respectively, of anoffset crest chisel with a negative draft;

[0023] FIGS. 8A-C are top, front and side views, respectively, of anoffset crest chisel with a negative draft and a reinforcement rib;

[0024] FIGS. 9A-C are top, front and side views, respectively, of anoffset conical insert with a negative draft;

[0025] FIGS. 10A-C are top, front and side views, respectively, of abiased negative draft chisel insert;

[0026] FIGS. 11A-C are top, front and side views, respectively, of apartial biased negative draft chisel insert;

[0027] FIGS. 12A-C are top, front and side views, respectively, of anarc crest chisel insert with zero draft;

[0028] FIGS. 13A-C are top, front and side views, respectively, of anarc crest chisel insert with negative draft;

[0029] FIGS. 14A-C are top, front and side views, respectively, of aspline or S-shaped crest chisel insert with zero draft;

[0030] FIGS. 15A-C are top, front and side views, respectively, of aspline or S-shaped crest chisel insert with negative draft;

[0031] FIGS. 16A-C are top, front and side views, respectively, of apartial negative draft chisel insert;

[0032] FIGS. 17A-C are top, front and side views, respectively, of anoffset crest chisel insert with negative draft on its leading flank;

[0033] FIGS. 18A-C are top, front and side views, respectively, of aslant crest chisel insert with negative draft;

[0034]FIG. 19 is a simplified illustration of a prior art insertpressing technique;

[0035]FIG. 20 is a simplified illustration of an insert pressingtechnique in accordance with the present invention;

[0036] FIGS. 21A-B are top and side views, respectively, of a row ofprior art steel teeth;

[0037] FIGS. 22A-B are top and side views, respectively, of a row ofprior art steel teeth having radiused crests;

[0038] FIGS. 23A-B are top and side views, respectively, of a row ofsteel teeth having negative draft in accordance with the presentinvention;

[0039] FIGS. 24A-B are top and side views, respectively, of a row ofsteel teeth having negative draft and radiused crests in accordance withthe present invention;

[0040] FIGS. 25A-B are top and side views, respectively, of a row ofbiased steel teeth having negative draft in accordance with the presentinvention;

[0041] FIGS. 26A-B are top and side views, respectively, of a row ofsteel teeth having partial negative draft in accordance with the presentinvention;

[0042] FIGS. 27A-B are top and side views, respectively of a steel toothhaving an offset crest and negative draft in accordance with the presentinvention;

[0043]FIG. 28 is a layout showing a first configuration of the cutterelements of the present invention with respect to a projection of theroller cone axis;

[0044]FIG. 29 is a layout showing an alternative configuration of thecutter elements of the present invention with respect to a projection ofthe roller cone axis;

[0045]FIG. 30 is a layout showing a second alternative configuration ofthe cutter elements of the present invention with respect to aprojection of the roller cone axis;

[0046]FIG. 30A is a different view of the configuration of FIG. 30,looking along the axis of the cutter element and showing its orientationwith respect to a projection of the cone axis;

[0047]FIG. 31 is a layout showing a third alternative configuration ofthe cutter elements of the present invention with respect to aprojection of the roller cone axis;

[0048]FIG. 32 is a profile of a single prior art steel tooth;

[0049]FIG. 33 is a profile of a first embodiment of a single steel toothconstructed in accordance with the present invention;

[0050]FIG. 34 is a profile of a single prior art radiused steel tooth;

[0051]FIG. 35 is a profile of a single radiused steel tooth constructedin accordance with the present invention;

[0052]FIG. 36 is a profile of a single prior art inverted radius steeltooth;

[0053]FIG. 37 is a profile of a single inverted radius steel toothconstructed in accordance with the present invention;

[0054]FIG. 38 is a profile of a single steel tooth having a partialnegative draft and constructed in accordance with the present invention;

[0055]FIG. 39 is a profile of an eight row tungsten carbide insert bitshowing inserts constructed in accordance with the present inventionrotated into a single plane;

[0056]FIG. 40 is a profile of a steel tooth bit showing teethconstructed in accordance with the present invention rotated into asingle plane; and

[0057]FIG. 41 is a profile of a steel tooth bit showing conventionalsteel teeth rotated into a single plane.

[0058] While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and are described in detail below. It should beunderstood, however, that the drawings and detailed description thereofare not intended to limit the invention to the particular formdisclosed, but on the contrary, the intention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0059] Referring first to FIG. 1, an earth-boring bit 10 made inaccordance with the present invention includes a central axis 11 and abit body 12 having a threaded section 13 on its upper end for securingthe bit to the drill string (not shown). Bit 10 has a predetermined gagediameter as defined by three rolling cone cutters 14, 15, 16, which arerotatably mounted on bearing shafts that depend from the bit body 12.Bit body 12 is composed of three sections or legs 19 (two shown inFIG. 1) that are welded together to form bit body 12. Bit 10 furtherincludes a plurality of nozzles 18 that are provided for directingdrilling fluid toward the bottom of the borehole and around cutters14-16. Bit 10 further includes lubricant reservoirs 17 that supplylubricant to the bearings of each of the cutters.

[0060] Referring now to FIG. 2, in conjunction with FIG. 1, each cutter14-16 is rotatably mounted on a pin or journal 20, with an axis ofrotation 22 orientated generally downwardly and inwardly toward thecenter of the bit. Drilling fluid is pumped from the surface throughfluid passage 24 where it is circulated through an internal passageway(not shown) to nozzles 18 (FIG. 1). Each cutter 14-16 is typicallysecured on pin 20 by ball bearings 26. In the embodiment shown, radialand axial thrust are absorbed by roller bearings 28, 30, thrust washer31 and thrust plug 32; however, the invention is not limited to use in aroller bearing bit, but may equally be applied in a friction bearingbit. In such instances, the cones 14, 15, 16 would be mounted on pins 20without roller bearings 28, 30. In both roller bearing and frictionbearing bits, lubricant may be supplied from reservoir 17 to thebearings by apparatus that is omitted from the figures for clarity. Thelubricant is sealed and drilling fluid excluded by means of an annularseal 34. The borehole created by bit 10 includes sidewall 5, cornerportion 6 and bottom 7, best shown in FIG. 2. Referring still to FIGS. 1and 2, each cutter 14-16 includes a backface 40 and nose portion 42spaced apart from backface 40. Cutters 14-16 further include afrustoconical surface 44 that is adapted to retain cutter elements thatscrape or ream the sidewalls of the borehole as cutters 14-16 rotateabout the borehole bottom. Frustoconical surface 44 will be referred toherein as the “heel” surface of cutters 14-16, it being understood,however, that the same surface may be sometimes referred to by others inthe art as the “gage” surface of a rolling cone cutter.

[0061] Extending between heel surface 44 and nose 42 is a generallyconical surface 46 adapted for supporting cutter elements that gouge orcrush the borehole bottom 7 as the cone cutters rotate about theborehole. Conical surface 46 typically includes a plurality of generallyfrustoconical segments 48 generally referred to as “lands” which areemployed to support and secure the cutter elements as described in moredetail below. Grooves 49 are formed in cone surface 46 between adjacentlands 48. Frustoconical heel surface 44 and conical surface 46 convergein a circumferential edge or shoulder 50. Although referred to herein asan “edge” or “shoulder,” it should be understood that shoulder 50 may becontoured, such as by a radius, to various degrees such that shoulder 50will define a contoured zone of convergence between frustoconical heelsurface 44 and the conical surface 46.

[0062] In the embodiment of the invention shown in FIGS. 1 and 2, eachcutter 14-16 includes a plurality of wear resistant inserts 60, 70, 80that include generally cylindrical base portions that are secured byinterference fit into mating sockets drilled into the lands of the conecutter, and cutting portions connected to the base portions havingcutting surfaces that extend from cone surfaces 44, 46 for cuttingformation material. The present invention will be understood withreference to one such cutter 14, cones 15, 16 being similarly, althoughnot necessarily identically, configured.

[0063] Cone cutter 14 includes a plurality of heel row inserts 60 thatare secured in a circumferential row 60 a in the frustoconical heelsurface 44. Cutter 14 further includes a circumferential row 70 a ofnestled gage inserts 70 secured to cutter 14 in locations along or nearthe circumferential shoulder 50 to cut the borehole wall. Cutter 14further includes a plurality of primary bottom hole cutting inserts 80,81, 82, 83 secured to cone surface 46 and arranged in spaced-apart innerrows 80 a, 81 a, 82 a, 83 a, respectively. Relieved areas or lands 78(best shown in FIG. 1) are formed about nestled gage cutter elements 70to assist in mounting inserts 70. As understood by those skilled in thisart, heel inserts 60 generally function to scrape or ream the boreholesidewall 5 to maintain the borehole at full gage and prevent erosion andabrasion of heel surface 44. Cutter elements 81, 82 and 83 of inner rows81 a, 82 a, 83 a are employed primarily to gouge and remove formationmaterial from the borehole bottom 7. Inner rows 80 a, 81 a, 82 a, 83 aare arranged and spaced on cutter 14 so as not to interfere with theinner rows on each of the other cone cutters 15, 16.

[0064] It is common for some of the cutter elements to be arranged onconical surface 46 so as to “intermesh” with each other. Morespecifically, performance expectations require that the cone bodies beas large as possible within the borehole diameter so as to allow use ofthe maximum possible bearing size and to provide adequate recess depthfor cutter elements. To achieve maximum cone cutter diameter and stillhave acceptable insert protrusion, some of the rows of cutter elementsare arranged to pass between the rows of cutter elements on adjacentcones as the bit rotates. In some cases, certain rows of cutter elementsextend so far that clearance areas corresponding to these rows areprovided on adjacent cones so as to allow the primary cutter elements onadjacent cutters to intermesh farther. The term “intermesh” as usedherein is defined to mean overlap of any part of at least one primarycutter element on one cone cutter with the envelope defined by themaximum extension of the cutter elements on an adjacent cutter.

[0065] Referring now to the particular construction of cutter elements,a prior art chisel insert 90 is shown in FIGS. 3A-D and a prior artconical insert 92 is shown in FIGS. 4A-C. As shown in these figures, theentire cutting portion of the insert is contained within the envelope ofthe cylindrical base portion. This is because the conventional way ofmanufacturing these inserts is by a punch and die method, which requirespositive draft at the cutting portion so as to allow the die halves toseparate after pressing operations. This restriction in manufacturingprocess imposes limitations on the geometry of the cutting portion ofthe insert. These limitations in turn prevent the optimization of thisgeometry for maximizing the bottom hole coverage and scraping actionneeded to increase rate of penetration in soft formations. Typicalpositive draft angles utilized in the manufacturing of these inserts arenot less than 10 degrees as measured per side, as shown in FIGS. 3B and4B.

[0066] The drawings show bases that are generally cylindrical, with somebeing of circular cross-section and some being non-circular (e.g. ovalor elliptical). However, the bases may be of any convenientcross-sectional shape and need not be cylindrical. While the followingdiscussion and corresponding Figures relate to cutter inserts havingcylindrical bases, it will be understood that the principles of thepresent invention can be applied with equal advantage to cutter insertshaving non-cylindrical bases. In cutter elements having non-circular orcylindrical bases, “positive draft” refers to instances where the entirecutting portion of the insert is contained within the envelope definedby projecting the shape of the base portion along the longitudinal axisof the cutter element. As used herein, the term “longitudinal axis”refers to the longitudinal axis of the base portion.

[0067] Referring now to FIGS. 5A-C, the chisel insert 100 of the presentinvention having an expanded geometry provides for increased mechanicalscraping/shearing action by providing increased crest length beyond thatformed on prior art inserts manufactured using conventionalmanufacturing techniques. Insert 100 includes base 102 and cuttingportion 104. The insert axis is shown as “a.” Further optimization ofmechanical scraping/shearing action can be achieved with additionalexpansion of cutting portion geometry as shown in FIGS. 6A-D. As shownin FIGS. 6A-D, insert 110 has a non-circular base 112 and cuttingportion 114 which includes expanded crest 116. Using the terminologyemployed with conventional manufacturing means, this novel insert has anegative draft 114, on the cutting portion which extends beyond theenvelope “e” of the cylindrical base portion. It is preferably made bythe manufacturing techniques described below.

[0068] Conventional roller cone drill bits generate an uncut area on thebore hole bottom known in the art as uncut bottom as shown in FIG. 3E.In FIG. 3E, the cutter elements from all rolling cone cutters aredepicted in rotated profile, that is, with the cutting profiles of thecutter elements shown as they would appear if rotated into a singleplane. The uncut bottom is the area on the bore hole bottom that is notcontacted by the crests of the primary row cutter elements. If thisuncut area is allowed to build up, it forms a ridge. In some drillingapplications this ridge is never realized, because the formationmaterial is easily fractured and the ridge tends to break off. In verysoft rock formations that are not easily fractured, however, theformation yields plastically and the ridge builds up. This ridgebuild-up is detrimental to the cutter elements and slows the drill bit'srate of penetration. Ridges of rock left untouched by conventionalcutting structure arrangements are reduced or eliminated by the use ofthe present invention as illustrated in FIG. 6E. FIG. 6E shows thereduction in uncut bottom or increased bottom hole coverage provided bythe expanded crest geometry of the cutter elements of the presentinvention.

[0069] To obtain the same degree of bottom hole coverage shown in FIG.6E using conventional cutter elements, the diameter of the base portionof the cutter elements would typically be increased to achieve thecorresponding increase in crest width. This increase in insert diameterwould have the result of reduced clearance between inserts in the samerow, as well as decreased insert-to-insert clearances between adjacentcones. To achieve adequate clearances in these areas would requiresevere compromise in insert count and placement. These compromises areavoided through the use of the present invention.

[0070] This invention is particularly suited for cutter elements used inthe primary rows where, in soft formations, maximum shearing andscraping action of the rock is the preferred method of cutting. Cutterelements with elongated crests are used in these formations to provideshearing capability. The crest width of these cutter elements insertsinfluences the aggressiveness of the cutting action relative to theformation. Thus, the function of expanded crest widths on an insert madein accordance with the. principles of the present invention can increasethe volume of shearing/scraping performed by the cutter element relativeto a conventional prior art chisel insert.

[0071] Hard formations can also be addressed by this invention.Increased cutter volume can be attained by expanding the insertextension beyond the base while maintaining effective clearances betweencutter elements in adjacent positions in the same row and betweenelements in adjacent rows (both on the same cone and in differentcones). With an expanded insert extension and a reduced base diameter,insert quantities can be increased, thereby providing greater cutterdensity with additional strikes to the formation. The increase in cutterdensity also provides additional wear time for the insert, therebyextending bit life.

[0072] Depending on the shape and/or orientation of the cutter element,bottom hole coverage can be maximized to reduce or eliminate the amountof uncut hole bottom. If the cutter elements are positioned to maximizebottom hole coverage, the number of bit revolutions necessary to gougeand scrape the entire hole bottom can be reduced 40-60% from a typicalconventional 3-cone tungsten carbide insert (TCI) rock bit.

Cutter Element Shapes

[0073] There are numerous variations within this invention for theconfiguration of the cutting portion of the insert that extend beyondthe envelope of the base portion. The geometry of the cutting elementcan be sculptured or non-sculptured. As used herein, the terms“contoured,” “sculpted” and “sculptured” refer to cutting surfaces thatcan be described as continuously curved surfaces wherein relativelysmall radii (typically less than 0.080 inches) are not used to breaksharp edges or round-off transitions between adjacent distinct surfacesas is typical with many conventionally-designed cutter elements. Thecutting portion of the cutting element can extend up to and beyond theenvelope of its base anywhere along the perimeter of the base portionand any multitude of times. The preferred manufacturing techniquesdescribed below allow for new insert shapes that extend up to and beyondthe “envelope” of the base portion of the insert thereby opening thedoor for countless new geometries. Several embodiments of the inventionas applied to insert type cutter elements are illustrated in FIGS. 5through 18. Like the embodiments shown in FIGS. 5A-C, 6A-D, theseembodiments incorporate the principles of the present invention. Foreach embodiment in FIGS. 7 through 18, the comments in Table I set outthe mechanical advantages that are believed to result from the specificfeatures of that embodiment. TABLE I FIG. Number Insert DescriptionComment Offset crest chisel with negative Optimize aggressive scrapingaction in draft. specific applications. Offset crest chisel withnegative The reinforcement rib provides draft and reinforcement rib.increased support to improve durability when drilling through hardstringers. Offset conical with negative Optimize scraping action innon-plastic draft. formations. Biased negative draft chisel. Optimizescraping action where insert - to - insert clearances between cones isconstrained. Partial biased negative draft Optimize scraping actionwhere insert to chisel. insert clearances between cones is constrained.Arc crest chisel with zero draft. Structural support for insertcrest/corners and improved scraping action. Arc crest chisel withnegative Structural support for insert crest/corners draft. andoptimized scraping action. Spine crest chisel with zero Structuralsupport for insert crest/corners draft. and improved scraping action.Spine crest chisel with negative Structural support for insertcrest/corners draft. and optimized scraping action. Partial negativedraft chisel. Insert chisel crest corner protection for tougherapplications. Offset crest chisel with negative Aggressive positive rakefor maximum draft on leading flank. formation removal. Slant crestchisel with negative Increased unit load upon entering the draft.formation to maximize penetration.

Cutter Element Placement

[0074] Further optimization of the cutter elements of the presentinvention can be achieved by their orientation and placement within thecone bodies. This will further maximize the desired level of scrapingaction for increased mechanical efficiency.

[0075] Referring to FIG. 28, novel inserts 110 are shown placed ina.conventional orientation in a row 110 a with the axis of each insertbeing coplanar with the cone axis. Another arrangement is shown in FIG.29, in which each insert 110 is oriented in the cone body such that theaxis “a” of the cylindrical portion of the insert is offset a distance“D” with respect of the cone axis. This further gives the designerflexibility to optimize the scraping action with regards to the specificformation and application.

[0076]FIGS. 30 and 30A show another orientation wherein the crest 116 ofthe insert 110 is rotated about the insert axis “a” such that an angle αis formed with respect to the projection of the cone axis. It will beunderstood that in certain applications, it may be advantageous torotate one or more inserts in the opposite direction such as by anamount α′. FIG. 31 shows another embodiment wherein the insert 110 isboth offset a distance “D” and rotated about its axis “a.” Any of theinserts shown in FIGS. 5-18 may be employed in the arrangements ororientations shown in FIGS. 28-31. The cutter elements 110 can bemechanically or metallurgically secured in the cone by various methods,such as, interference fit, brazing, welding, molding, casting, orchemical bonding. The inserts described in the FIGS. 5 and 7-18 andorientations 28-31 are shown with a cylindrical base portion forinterference fit into a matching socket. It will be understood negativedraft does not require that the base portion be cylindrical, but doesrequire that the cutting portion of the insert extend up to or beyondthe noncylindrical envelope defined by the base portion, as shown inFIGS. 6A-D.

Insert Material Types

[0077] An insert of the present invention can be made of tungstencarbide and in addition can be partially or fully coated with a“superabrasive” (i.e., a material having a hardness of at least 2,700Knoop kg/mm2) such as PCD, PCBN, etc.

[0078] Conventional rolling cone bit inserts are manufactured by pressand die operations. As shown in FIG. 19, the top and bottom dies 8, 3are pressed axially with respect to the longitudinal axis “a” of insert1, to form an insert 1 with a cylindrical base 9 and an extendingportion 2, contained within the envelope of the cylindrical base.Positive draft must be provided so as to keep extending portion 2 withinthe constraints of the cylindrical base. Draft refers to the taper givento internal sides of a closed-die to facilitate its removal from the diecavity. To complete the conventional insert 1, a centerless grindoperation is performed on the base portion 9 to provide specifiedcylindrical geometry and surface finish. In centerless grinding theinsert 1 is supported on a work rest and fed between the grinding wheeland a rubber bonded abrasive regulating wheel. Guides on either side ofthe wheels direct the work to and from the wheels in a straight line.

[0079] When inserts have extending geometries that extend out to andbeyond the envelope of the cylindrical base as contemplated by thepresent invention, conventional manufacturing techniques such as axialinsert pressing and centerless grinding cannot be used. Techniques havebeen and are being developed to provide the ability to create the novelinserts of the present invention such as those shown in FIGS. 5-18. Forexample, instead of pressing each insert along the longitudinal axis ofits base “a,” the inserts of the present invention (such as insert 110of FIGS. 6A-D) can be pressed normal to that axis, as shown in FIG. 20,thus creating sides instead of a top and bottom. The present insert 110can also be manufactured by injection molding, multi-axis CNC millingmachine, wire EDM, casting, stereolithography or other free-formingmethods.

[0080] The insert base portion 112 can be finished by using othergrinding methods (post grinder, in-feed centerless grinder) or by singlepoint machining (turning).

Other Applications for Invention

[0081] Application of this invention is not restricted to use on therolling cones of insert bits. The cutter elements can be used on theprimary rows of big hole cutters and the bottom. hole cutting elementsof hammer bits. Further, the advantages of this invention are notlimited to inserts or compacts, but can be equally applied to teeth of asteel tooth bit.

[0082] Steel tooth bits typically have teeth that are milled, cast orotherwise integrally formed from the base material or parent metal ofthe cone. FIGS. 21A-B depict a portion of a rolling cone cutter of asteel tooth bit. Specifically, FIGS. 21A-B depict a row 120 a havingsteel teeth 120. The other inner rows of steel teeth of this cone cutterare not shown in these figures. The profile of steel tooth 120 is bestshown in FIG. 32. Tooth 120 is depicted in FIGS. 21A-B without hardfacing, a hard, durable metal coating that is applied to the parentmetal of tooth 120 to increase its durability. The hard facing 120 h andparent metal 120 p of tooth 120 are shown in FIG. 32. As shown, theparent metal of conventional tooth 120 includes crest 122 having crestlength (CL) and a root 124 with a root length (RL) that is greater than(CL).

[0083] FIGS. 22A-B disclose a row 126 a of steel teeth 126 of a priorart cone cutter. FIG. 34 discloses a profile view of tooth 126. Asshown, tooth 126 includes a crest 128 having recess 130 and root 132. Aswith tooth 120, tooth 126 includes a root 132 having root length (RL)greater than the crest length (CL) of crest 128. The crest 128 havingrecess 130 is referred to herein as a radiused crest steel tooth.

[0084]FIG. 36 shows a profile view of another prior art tooth 140similar to teeth 120, 126 previously described. Tooth 140 includes crest142, sides 144 and root 146. The comers of the tooth 140 at theintersection of sides 144 and crest 142 have an inverted radius at 148.

[0085] On conventional steel tooth bits, the width of the cuttingportion of the parent metal of the tooth is smaller than the width ofits base. More specifically, the crest length (CL) is less than the rootlength (RL) of the tooth for a conventional steel tooth as best shown inFIGS. 32, 34 and 36. By contrast, in this invention, the width of thecutting portion of the tooth can be larger than the base, beforehardfacing is applied, as shown in FIGS. 33, 35 and 37. Although thesteel tooth does not have a cylindrical base portion with a cuttingportion extending beyond this base portion, the cutting portion doeshave a substantially wider crest length than the root length ofconventional bits. This wider crest length, and the increased bottomhole coverage it provides, maximizes the scraping and shearing action onthe formation, thus significantly improving the penetration rate of thebit. Several variations of steel teeth designed according to theprinciples of the present invention are described below and illustratedin FIGS. 23 through 27. For each embodiment in FIGS. 23 through 27, thecomments in Table II describe the mechanical advantages that arebelieved to result from the specific features of that embodiment. TABLEII Negative draft steel Increased mechanical scraping/shearing actiontooth. due to increased crest length beyond prior art steel teeth.Negative draft steel Similar to FIG. 21, but employing the benefitstooth with radiused of the radiused or rounded corners to enhance crest.the retention of hardfacing onto the tooth (as described in SmithInternational patent 5,152,194). Biased negative draft Optimize scrapingaction where tooth-to-tooth steel tooth. clearances between cones isconstrained. Partial negative draft Tooth crest corner protection fortougher steel tooth. applications. Offset crest steel Optimizeaggressive scraping action in specific tooth with negative applications.draft.

[0086] In the more detailed description that follows, the steel teeth ofthe invention will be described and depicted without hardfacing, itbeing understood that hardfacing could, and in many applications would,be applied over the parent metal of the tooth.

[0087] One embodiment of the present invention employed in a steel toothbit is shown in FIGS. 23A, 23B and 33. The rolling cone cutter includesa row 200 a of steel teeth 200. As best shown in FIG. 33, tooth 200includes a crest 202 and root 204. Crest length (CL) of crest 202 isgreater than root length (RL) of root portion 204.

[0088] Another embodiment of the present invention is shown in FIGS.24A, 24B and 35. As shown, the rolling cone cutter includes a row 206 aof radiused crest steel teeth 206. As best shown in the profile view ofFIG. 35, tooth 206 includes crest 208 and root portion 210. Crest 208includes a recess 212. Tooth 206 is formed such that crest length (CL)is greater than root length (RL) in accordance with the principles ofthe present invention.

[0089] Another embodiment of the present invention is shown in FIG. 37.FIG. 37 is a profile view of a steel tooth similar to that shown inFIGS. 33 and 35. In the embodiment shown in FIG. 37, tooth 220 includescrest 222 and root 224. The crest length (CL) of crest 222 is greaterthan root length (RL) of root 224. The comers of tooth 220 formed at theintersection of crest 222 and sides 226 includes a portion 228 having aninverted radius. In this embodiment, the crest length is measuredbetween the points of intersection formed by extensions of the crest 222and sides 226 as shown in FIG. 37. Similarly, the root length ismeasured between the intersections of the extensions of sides 226 andcone surface 230.

[0090] Referring to FIGS. 25A-B, another embodiment of the presentinvention is applied to a steel tooth bit. As shown, the steel toothcone cutter includes a row 240 a of steel teeth 240. Each tooth 240includes a crest 242 and a root portion 243. Crest 242 intersects sides244, 246 in angles θ₁ and θ₂, respectively. As shown, θ₁ is an anglegreater than 90°, while θ₂ is an angle less than 90°. The crest length(CL) of crest 242 is greater than the root length (RL) of root 243.Although in this particular embodiment, θ₁ is greater than θ₂, theinvention is not limited to this or any other relationship for θ₁ andθ₂. Likewise, the crest can take various forms such as a rounded crestor non-linear crest, but the intent is that the overalllinearly-measured width of the crest exceeds that of the root.

[0091] Another embodiment of the invention is shown in FIGS. 26A-B, andFIG. 38. As shown, a steel tooth cone cutter includes a row 250 a ofsteel teeth 250. Each tooth 250 includes a crest 252, root portion 254,a pair of upper sides 258 and a pair of lower sides 259. Theintersection of each upper side 258 and lower side 259 forms a centralportion having an expanded length EL that is greater than root length(RL) and, in this embodiment, greater than crest length (CL). The rootlength (RL) is measured from the intersections of the extensions oflower sides 259 and cone surface 260.

[0092]FIGS. 27A and 27B shown an embodiment similar to that depicted inFIGS. 23A, 23B; however, the embodiment shown in FIGS. 27A, 27B isformed such that crest 202 is offset a distance D from a line that isparallel to crest 202 and that passes through the axis 300 of the cone.Crest orientations similar to TCI FIGS. 30 and 31 can also be applied tosteel tooth designs.

BIT DESIGN INTENT

[0093] Depending on the bit design objectives, the amount of uncutbottom can be reduced or eliminated. Currently, most bits are designedwith cutter intermesh between the rolling cones, which can invokelimitations on the wider crest of the cutter elements. Hence, designingbits without intermesh can allow greater latitude in crest width.

[0094] Additionally, these cutter elements can be used in all types ofrolling cone bits having one, two or more rolling cones.

[0095] The increased bottom hole coverage attainable with the presentinvention permits the use of fewer rows of cutter elements on the conecutters of the bit. Having fewer rows of cutter elements, as compared toconventional prior art bits, increases the unit loading per cutterelement thus increasing rate of penetration. For example, in oneconventional 3-cone TCI roller cone bit, a total of nine rows of primarycutter elements were dispersed among the three cones employed to cut thebottom hole as shown in rotated profile in FIG. 3E, there being threerows, specifically Rows 7, 8 and 9, aligned in the same rotated profileposition. Using the expanded crest geometry of the present invention,and as shown in rotated profile FIG. 39, the bottom hole coverage can beattained using only a total of 8 rows of cutter elements on this 3-conebit. Thus, the present invention allows TCI bits to be designed with 8or fewer rows, in contrast to conventional prior art TCI bits, whichtypically have 9 or more rows.

[0096] Similarly, prior art steel tooth bits such as that shown inrotated profile in FIG. 41 typically included a total of seven rows ofcutter elements for bottom hole coverage. Use of the present invention,as shown in FIG. 40, permits bottom hole coverage to be attained usingonly six rows of cutter elements made in accordance with the presentinvention. Thus, the present invention allows steel tooth bits to bedesigned with 6 or fewer rows, in contrast to conventional prior artsteel tooth bits, which typically have 7 or more rows.

[0097] While various preferred embodiments of the invention have beenshown and described, modifications thereof can be made by one skilled inthe art without departing from the spirit and teachings of theinvention. The embodiments described herein are exemplary only, and arenot limiting. For example, the present invention includes cutterelements having shapes other than the shapes shown and described herein.Many variations and modifications of the invention and apparatusdisclosed herein are possible and are within the scope of the invention.Accordingly, the scope of protection is not limited by the descriptionset out above, but is only limited by the claims that follow, that scopeincluding all equivalents of the subject matter of the claims.

What is claimed is:
 1. A drill bit comprising: a bit body; at least oneroller cone rotatably mounted on a cantilevered bearing shaft dependingfrom said bit body; and at least one cutter element extending from aprimary row in said roller cone, said cutter element having a baseportion adapted to fit into a corresponding socket on said roller coneand a non-rectilinear crest.
 2. The bit in accordance with claim 1wherein said crest is arcuate.
 3. The bit in accordance with claim 1wherein said crest is a substantially S-shaped spline.
 4. The bit inaccordance with claim 1 wherein said cutter element has a leading facethat includes both concave and convex portions when viewed along thelongitudinal axis of the cutter element.
 5. The bit in accordance withclaim 1 wherein said cutter element has a concave leading face and aconvex trailing face when viewed along the longitudinal axis of thecutter element.
 6. The bit in accordance with claim 1 wherein said baseis non-circular
 7. The bit in accordance with claim 1 wherein said baseis non-cylindrical.
 8. The bit in accordance with claim 1 wherein atleast two cutter elements have non-rectilinear crests.
 9. The bit inaccordance with claim 1 wherein at least one cutter element has anextending portion having zero draft.
 10. The bit in accordance withclaim 1 wherein at least one cutter element has an extending portionhaving negative draft.
 11. The bit in accordance with claim 1 wherein atleast one cutter element has an extending portion having positive draft.12. The bit in accordance with claim 1 wherein at least one cutterelement has an extending portion having a contoured surface.
 13. Thedrill bit according to claim 1 wherein said cutter element has alongitudinal axis and said longitudinal axis is offset such that it doesnot intersect the axis of said cone.